The insufficiency of nature to such a work as conversion is, shows that men may not fall down and idolize their own wit and power. A change from acts of sin to moral duties may be done by a natural strength and the power of natural conscience: for the very same motives which led to sin, as education, interest, profit, may, upon a change of circumstances, guide men to an outward morality; but a change to the contrary grace is supernatural.

Two things are certain in nature.

(1.) Natural inclinations never change, but by some superior virtue. A loadstone will not cease to draw iron, while that attractive quality remains in it. The wolf can never love the lamb, nor the lamb the wolf; nothing but must act suitably to its nature. Water cannot but moisten, fire cannot but burn.

So likewise the corrupt nature of man being possessed with an invincible contrariety and enmity to God, will never suffer him to comply with God. And the inclinations of a sinner to sin being more strengthened by the frequency of sinful acts, have as great a power over him, and as natural to him, as any qualities are to natural agents: and being stronger than any sympathies in the world, cannot by a man's own power, or the power of any other nature equal to it, be turned into a contrary channel.

(2.) Nothing can act beyond its own principle and nature. Nothing in the world can raise itself to a higher rank of being than that which nature has placed it in; a spark cannot make itself a star, though it mount a little up to heaven; nor a plant endue itself with sense, nor a beast adorn itself with reason; nor a man make himself an angel. Thorns cannot bring forth grapes, nor thistles produce figs because such fruits are above the nature of those plants. So neither can our corrupt nature bring forth grace, which is a fruit above it. Effectus non excedit virtutem suae causae [the effect cannot exceed the power of its cause]: grace is more excellent than nature, therefore cannot be the fruit of nature.

It is Christ's conclusion, "How can you, being evil, speak good things?" Matt. 12:33, 34. Not so much as the buds and blossoms of words, much less the fruit of actions. They can no more change their natures, than a viper can do away with his poison. Now though this I have said be true, yet there is nothing man does more affect in the world than a self-sufficiency, and an independence from any other power but his own. This attitude is as much riveted in his nature, as any other false principle whatsoever. For man does derive it from his first parents, as the prime legacy bequeathed to his nature: for it was the first thing uncovered in man at his fall; he would be as God, independent from him.

Now God, to cross this principle, allows his elect, like Lazarus, to lie in the grave till they stink, that there may be no excuse to ascribe their resurrection to their own power. If a putrefied rotten carcass should be brought to life, it could never be thought that it inspired itself with that active principle. God lets men run on so far in sin, that they do unman themselves, that he may proclaim to all the world, that we are unable to do anything of ourselves towards our recovery, without a superior principle. The evidence of which will appear if we consider,

1. Man's subjection under sin. He is "sold under sin," Rom. 7:14, and brought "into captivity to the law of sin," ver. 23. "Law of sin:" that sin seems to have a legal authority over him; and man is not only a slave to one sin, but many, Tit. 3:3, "serving divers lusts." Now when a man is sold under the power of a thousand lusts, every one of which has an absolute tyranny over him, and rules him as a sovereign by a law; when a man is thus bound by a thousand laws, a thousand cords and fetters, and carried whither his lords please, against the dictates of his own conscience and force of natural light; can any man imagine that his own power can rescue him from the strength of these masters that claim such a right to him, and keep such a force upon him, and have so often baffled his own strength, when he attempted to turn against them?

2. Man's affection to them. He does not only serve them, but he serves them, and every one of them, with delight and pleasure; Tit. 3:3. They were all pleasures, as well as lusts; friends as well as lords. Will any man leave his sensual delights and such sins that please and flatter his flesh? Will a man ever endeavour to run away from those lords which he serves with affection? having as much delight in being bound a slave to these lusts, as the devil has in binding him.

Therefore when you see a man cast away his pleasures, deprive himself of those comfortable things to which his soul was once knit, and walk in paths contrary to corrupt nature, you may search for the cause anywhere, rather than in nature itself. No piece of dirty, muddy clay can form itself into a neat and handsome vessel; no plain piece of timber can fit itself for the building, much less a crooked one. Nor a man that is born blind, give himself sight.

God deals with men in this case as he did with Abraham. He would not give Isaac while Sarah's womb, in a natural probability, might have borne him; but when her womb was dead, and age had taken away all natural strength of conception, then God gives him; that it might appear that he was not a child of nature, but a child of promise.